The present invention relates to high velocity burner firing of furnace combustion chambers and, more particularly, it relates to a burner and burner firing method and system by which the formation of nitrogen oxides (NO.sub.x) is reduced at substantially all levels of burner heating capacities.
Techniques for controlling and inhibiting NO.sub.x formation in furnace combustion processes are well known and may include, for example, provisions for staging fuel, staging combustion air, recirculating flue gas into the burner, recirculating flue gas into the burner flame, altering combustion patterns with different degrees of swirl, and injection of water or steam into the burner or flame. Factors which contribute to the formation of NO.sub.x in burner fired combustion chambers are the oxygen content of the flame or combustion chamber, the temperature of the combustion chamber and the burner firing rate. It is known that the NO.sub.x increases with combustion chamber temperature and with oxygen content in the combustion chamber. However, these factors are difficult to predict because burners for different industrial processes must operate at various furnace chamber temperatures, have various oxygen concentrations in the work chambers, and are required to operate at different heat inputs depending of changing heat load requirements.
Most modern industrially available burners that are known as "high velocity burners" are relatively low NO.sub.x producers because, at the higher firing rates of such burners, large amounts of combustion chamber or flue gasses are entrained into the burner flames. As a result, not only is localized high flame temperature reduced, but also, flue gas is directed into and mixed with the flame of the burning combustible mixture. This effect becomes less pronounced at reduced or low fire flow rates of fuel and air since there is less kinetic energy to entrain the furnace gasses into the flame and to stir the furnace work chamber flue gasses for best furnace temperature uniformity. In addition, flames at minimum flow rates also are usually smaller and do not occupy an adequate percentage of furnace chamber volume to ensure the induction of flue gasses into the flame to lower the formation of NO.sub.x.
In a commercially available, high velocity gas burner manufactured and sold by Hauck Manufacturing Co. of Lebanon, Pennsylvania, the assignee of the present invention, under the designation "Burner Model SVG 115," furnace combustion chamber temperatures developed by the burner are controlled through frequency modulation of burner firing between full capacity firing rates and pilot firing rates. Pilot firing rates, in this context, are those in which an adequate small amount of fuel and air is supplied to the burner for maintaining ignition but without development of meaningful furnace chamber heat. By such on/essentially-off operation, the kinetic energy of burning gases accelerated from the burner entrains flue gases into the burning gas and inhibits the formation of localized high temperature and/or oxygen-rich regions in the burning gases. As a result NO.sub.x formation is reduced substantially by comparison to continuous burner firing at varying rates of fuel and air supply for temperature control purposes.
It is also known that NO.sub.x formation can be reduced by staging the air supply to a gas burner in a manner so that mixture of fuel and a substoichiometric quantity of air is ignited and discharged for complete combustion supported by secondary air mixed with the burning gases downstream from the burner. An example of such a staged air supply gas burner is disclosed in U.S. Pat. No. 4,021,188 issued May 3, 1977 to Kazuo Yamagishi et al. While the disclosure of this patent includes many variations of nozzle structures for attainment of low NO.sub.x formation using staged burner air supply, only one mode of burner operation is described and no disclosure is made of controlling or varying the heating capacity of the burner.
The present invention has been made in view of the above circumstances and has as an object the provision of a high velocity burner construction by which the formation of NO.sub.x in a furnace combustion chamber fired by the burner is reduced throughout a wide range of furnace combustion chamber temperatures.
A further object of the present invention is to provide a system for the supply of fuel and air to such a burner.
Another object of the present invention is the provision of a method of operating such a system and burner.
Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the low NO.sub.x burner method and system of this invention comprises operating the burner in a first mode at furnace combustion chamber temperatures up to a transitional temperature by accelerating a burning mixture of fuel and air to moderately high velocities into the furnace combustion chamber to ensure a mixing of flue gases with the burning mixture of fuel and air, and operating the burner in a second mode at furnace combustion temperatures above the transitional temperature by introducing into the combustion chamber, a relatively low velocity stream of burning fuel mixed with a minor amount of air needed for stoichiometric combustion and accelerating a separate stream of air to high velocities into the furnace combustion chamber for mixture with the low velocity stream downstream from the burner in the furnace combustion chamber, the separate stream of air comprising the remainder of air required for stoichiometric combustion of the fuel.
The invention is further embodied in a high velocity burner system for furnace combustion chambers, the system including a burner having an ignition chamber for discharging an ignited combustible mixture of primary air and fuel into the furnace combustion chamber, and at least one nozzle port for directing a high velocity stream of secondary air into the furnace combustion chamber in a direction generally parallel to the direction of flow from the ignition chamber, means for supplying fuel to the ignition chamber, means for supplying primary air to the burner during plural modes of burner operation including a first mode during which primary air alone is supplied to the ignition chamber and a second mode during which primary air is a minor percentage of air used for combustion of fuel supplied to the burner, means for supplying to the combustion chamber, secondary air in amounts constituting a major percentage of air used for combustion of fuel during the second mode, means for regulating the fuel supplying means so that fuel supply to the burner is dependent on operation of one of the primary air supplying means alone and both the primary air supplying means and the secondary air supplying means together to supply air to the burner, and means for controlling the firing rate of the burner at least during the second mode including secondary valve means for intermittently terminating operation of the secondary air supplying means for variable periods of time.